1. Field of the Invention
The invention relates to a turbomachine with radial-flow compressor impeller, which is enclosed by a spiral-shaped compressor casing with a volute-shaped flow duct having an essentially axially deflecting section and an essentially radially deflecting section. The casing includes an outer spiral casing, which includes the radially deflecting section and is rigidly fixed to the bearing casing, and an inner casing insert piece having an inner contour which with the outer contour of the hub forms the axially deflecting section.
2. Description of the Related Art
The fundamental construction and the mode of operation of such turbomachines, such for example as a centrifugal compressor of a turbocharger, are known per se and therefore require no more detailed explanation in the present connection. Thus, for example, DE 195 02 808 C2 describes a generic turbomachine in the form of a centrifugal compressor of a turbocharger, within whose spiral-shaped compressor casing the diameter of the hub of the compressor impeller and of the vanes increases in the flow direction. The outer contour of the vanes is curved and corresponds to the inner contour of the adjacent, toroidal outwardly curved duct wall of the volute-shaped flow duct. The duct wall of the compressor casing, together with the outer contour of the hub of the compressor impeller, bounds an outwardly deflected duct section, in which the vanes engage. An annular duct section opening into a spiral duct abuts this radially outwardly deflected duct section.
Such compressor casings of turbochargers, and their inserts, are usually of rigid configuration.
After lengthy operation under unfavorable conditions, such a compressor impeller can be so greatly weakened by corrosion, erosion and aging that fracture of the compressor impeller cannot be excluded. In the event of a compressor impeller fracture, in which the impeller usually breaks into two or three large fragments, these individual parts are thrown outwards due to the substantial centrifugal forces. Fragments can then even emerge from the compressor casing. In the process, the compressor impeller vanes are completely destroyed and the remaining hub body jams between the bearing casing and the compressor casing. Due to the shaping of the hub pieces, a wedge effect then occurs, which hub pieces exert substantial impulse-type axial forces on the casing.
Smaller turbochargers can absorb these forces due to the relatively large wall thicknesses and the stiff casing parts. In the case of large turbochargers, the casing wall thicknesses are usually reduced for technical casting reasons so that, in the case of such loads, the fracture limit of the material is rapidly reached and casing fractures can occur. Fragments can then emerge from the turbocharger, with substantial consequential damage.
This is to be avoided at all costs. For this purpose, it is now usual to provide an additional burst protection arrangement outside the compressor casing which accommodates the compressor impeller.
On this basis, therefore, the object of the present invention is to so develop a turbomachine of the type mentioned at the beginning, using simple and low-cost means, that the emergence of fragments of a burst compressor impeller from the compressor casing can be avoided, without having to provide an additional burst protection arrangement outside the spiral casing.
According to the invention, the outer spiral casing includes an inner cylinder to which the inner casing piece is fixed by means of a fixing arrangement to form a hollow space between the inner cylinder and the insert piece. The fixing arrangement is axially flexible and is less secure against fracture than the rigid fixing of the outer spiral casing to the bearing casing.
Because the compressor casing has an outer spiral casing, which comprises the flow duct section, which is deflected outwards into the radial direction, and an inner casing insert piece, which is provided in the radial direction between the spiral casing and the compressor impeller and whose inner contour, together with the outer contour of the hub of the compressor impeller, forms the flow duct section, which extends essentially in the axial direction, because the spiral casing is configured with an inner cylinder at least partially surrounding the casing insert piece. The casing insert piece is attached on the inner cylinder by means of a fixing arrangement, which is flexible in the axial direction, to form a hollow space, and because the flexible fixing arrangement of the casing insert is configured to be less secure against fracture than the rigid fixing arrangement of the compressor casing on the bearing casing, a spiral casing is configured with a xe2x80x9ccrumple zonexe2x80x9d, from which no fragment of a burst impeller can now emerge. The kinetic energy of fragments of a bursting compressor impeller can now be completely converted into deformation energy and heat within the turbocharger.
The rigid fixing arrangement of the spiral casing on the bearing casing, and the casing insert piece itself, can absorb considerably more kinetic energy than the flexible fixing arrangement of the casing insert piece on the spiral casing. This ensures that the casing insert piece can, in an emergency, move away from the compressor impeller in the axial direction. In this way, the kinetic energy of the compressor impeller pieces can be largely absorbed by conversion into deformation energy and the heat resulting from it. The residual kinetic energy of the fragments can be absorbed by the casings.
It is possible to dispense with an additional burst protection arrangement outside the spiral casing.
The spiral casing is configured with an inner cylinder, which at least partially surrounds the casing insert piece, on which inner cylinder, the casing insert piece is attached, to form a hollow space, by means of a fixing arrangement which is flexible in the axial direction. This ensures a high level of protection of the spiral casing itself from impact and an effective xe2x80x9cbrakexe2x80x9d to combat any emergence of fragments from the turbocharger. In a particularly advantageous manner, the rigid fixing arrangement of the compressor casing is configured by means of a firm flange connection of the spiral casing to the bearing casing and the flexible fixing arrangement of the casing insert piece is configured by means of a reduced-shaft screw fixing arrangement in the axial direction through the inner cylinder of the spiral casing.
In a particularly advantageous manner, the bearing casing wall provided for the rigid fixing arrangement of the spiral casing is arranged so that it is drawn downwards in the radial direction to above the outer tip of the outer contour of the compressor impeller, while forming a gap, so that an additional xe2x80x9cbrakexe2x80x9d, i.e. security against radial emergence of fragments of the compressor impeller, is provided.
The compressor casing is preferably positioned by means of the outer spiral casing, while forming a joint on the bearing casing, in such a way that the outer spiral casing comprising the deflected duct section, is drawn inward in the radial direction B past the rigid fixing arrangement of the compressor casing on the bearing casing. The rigid fixing arrangement is arranged so that it is located further outward, in the radial direction B, than the joint between the spiral casing and bearing casing. This measure, comprising the particular arrangement of the joint, ensures that the rigid fixing arrangement of the compressor casing on the bearing casing hardly suffers any loads from fragments possibly becoming wedged in the flow duct, by which means, breakage of the compressor casing is excluded to an even greater extent.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.